The association between CD4+ T cell depletion and accelerated rate of disease progression is well established for HIV in humans and simian immunodeficiency virus (SIV) infection of Rhesus macaques. In contrast to pathogenic infections, SIV infection of certain African primate species (i.e. sooty mangabeys) does not result in progression to simian AIDS despite high levels of viral replication, initially this absence of disease progression was attributed to both lower immune activation and preserved CD4+ T cell levels. More recent studies have challenged the idea that maintenance of normal CD4+ T cell levels is a key factor that protects natural SIV hosts from simian AIDS. In 2007 the Sodora laboratory published a study in which plasma from a naturally SIV-infected mangabey with normal CD4+ T cell counts (above 600 cells/ul) was inoculated to six SIV-uninfected mangabeys in October 2000. Two of these experimentally infected mangabeys exhibited a dramatic CD4+ T cell decline in blood (<80 cells/ul) as well as lymph nodes and GALT. These SIV+ CD4-low mangabeys show no evidence of systemic immune activation during chronic infection. We determined that this dramatic CD4+ T cell depletion is virally mediated, as replicating virus has an expanded co-receptor utilization profile and passage of this virus to three mangabeys in October 2006 again resulted in dramatic CD4+ T cell depletion. To date, none of the SIV+ CD4-low mangabeys have exhibited any clinical signs of simian AIDS even though they have been CD4-low for between 3 to 8 years. The goal of this proposal is to ascertain the mechanisms by which SIV+ CD4-low mangabeys are able to maintain immunologic health. We hypothesize that the preserved immune system is due in part to the presence of double-negative (CD3+/CD4-/CD8-) T cells that are able to provide the necessary T helper function. The first aim will characterize the similarities between double-negative and CD4+ T cells in mangabeys, macaques, and humans, assessing both phenotype and function. The second aim will functionally assess the double-negative T cells from these three species with regard to T helper function;both B cell antibody class switching and regulatory T cell activity. The third aim will evaluate the ability of SIV-infected CD4-low mangabeys to elicit effective innate and adaptive immune responses to diverse vaccine challenges and to assess the role of double-negative T cells the vaccine specific immune responses. This unique cohort of animals allows for the innovative functional characterization of T cell populations that do not express CD4, have the potential to perform the function of helper T cells and are refractory to SIV infection. Importantly, our studies will also be assessing the double-negative T cells in Rhesus macaques and humans permitting a determination if these cells have potential to perform a similar task in HIV+ patients. Our findings could potentially lead to novel strategies for development of immune therapies or vaccines that induce functional activity from the double-negative T cell subset which is refractory to the HIV infection.